Please read this article it sounds they covered different protocols for activation of channelrhodopsin, however its in nervous cells, but you may optimise the protocol yourself to suit your experiment conditions
Hi. I would just like to add a suggestion- Do not use a very focused beam. Aside from phytotoxicity issues, you could also get false readings in the electrical readings. Basically, do consider how tightly your beam is getting focused, calculate beam spot size and then the intensity. This should be lower than the max you intend to go to. I faced some issues related to this when I was patching HEK cells. If the intensity is too high (either the power of beam is high, or the spot size is really small), it could give spurious reading in the electrical recording in addition to possible phototoxicity issues. Firsthand experience :D